This PhD thesis offers a broad picture and aims at building a step-wise understanding of some critical processes determining the fate and biodegradation of oil in deep marine oil spills and the influence of chemical dispersants as a clean-up response in each step. Deep marine injection of chemical dispersants, for example, in the Deepwater Horizon oil spill, puts the deep parts of the marine ecosystem in danger. The most pronounced effects are the intensive oil dissolution in the water column, oil components toxicity and stress impulses to the marine (micro) organisms, excessive formation of marine snow, and scavenging of oil particles from the water column to extensive oil sedimentation towards the seafloor. This new understanding generated in our studies suggests that it may be better to design clean-up strategies that are so-called “nature-based”, which means using naturally occurring processes that support the self-healing capacities of the environment.